Determination of nonylphenol ethoxylate metabolites in vegetables and crops by high performance liquid chromatography–tandem mass spectrometry

Food Chemistry 132 (2012) 502–507

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Analytical Methods

Determination of nonylphenol ethoxylate metabolites in vegetables and crops by high performance liquid chromatography–tandem mass spectrometry Yongxin She a,b, Jing Wang a,⇑, Yongquan Zheng c, Weiqiang Cao d, Rongyan Wang a, Fengshou Dong c, Xingang Liu c, Mingrong Qian e, Hu Zhang e, Liqing Wu e a

Institute of Quality Standards and Testing Technology for Agri-Products, Chinese Academy of Agricultural Science, 100081 Beijing, China Institute of Veterinary and Animal Husbandry, Tibetan Academy of Agricultural and Animal Husbandry, 850009 Lhasa, China Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Key Laboratory of Pesticide Chemistry and Application, Ministry of Agriculture, 100193 Beijing, China d Chinese Agricultural Products Monitoring Centre, Hangzhou 310012, Zhejiang, China e Huizhou Import and Export Inspection and Quarantine Bureau, 450012 Huizhou, China b c

a r t i c l e

i n f o

Article history: Received 11 May 2010 Received in revised form 22 May 2011 Accepted 28 September 2011 Available online 10 October 2011 Keywords: Nonylphenol polyethoxylates Metabolites Vegetables Crops LC–MS/MS

a b s t r a c t A method has been developed for the simultaneous determination of the concentration of nonylphenol (4-NP), nonylphenol monoethoxylates (NP1EO) and nonylphenol diethoxylates (NP2EO) in vegetables and crops by liquid chromatography–tandem quadrupole mass spectrometry (HPLC–MS/MS). These target compounds were extracted from vegetable and crop samples with acetonitrile, and then the extracts were cleaned using solid phase extraction with graphitised carbon black tandem primary secondary amine (PSA) cartridges. The MS method enabled highly reliable identification by monitoring the corresponding ammonium adduct [M+NH4]+ in the positive mode for NP1EO and NP2EO, and the deprotonated molecule [M H] in the negative mode for 4-NP. Recoveries for the spiked samples ranged from 65% to 118%. The limit of detection (LOD) of 4-NP, NP1EO and NP2EO was 3, 5 and 0.1 lg kg 1, respectively. This method would be useful for the quick and routine detection of the residues of 4-NP, NP1EO and NP2EO in vegetables and crops. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Nonylphenol polyethoxylates (NPnEOs) are among the most commonly used non-ionic surfactants because of their favourable physicochemical characteristics. They have been incorporated into detergents, emulsifiers, humidifiers, stabilizers, skimmers and intermediates in a great variety of industries (Fountoulakis et al., 2005; Langford, Scrimshaw, & Lester, 2007). Under certain conditions, they are biodegradable to shorter-chain compounds and more persistent products such as nonylphenol (4-NP), nonylphenol monoethoxylate (NP1EO) and nonylphenol diethoxylate (NP2EO) (Soares, Guieysse, Jefferson, Cartmell, & Lester, 2008). These degraded substances have a greater toxicity towards endocrine disruption and reproduction than that of their parent compounds (Belfroid et al., 1999; Berryman, Houde, DeBlois, & O’Shea, 2004; Routledge et al., 1998). As a result of the potential interference with estrogens by NPEO and bioaccumulation in organisms, NPEO was banned for use in detergents by the 2000 OSPAR convention (PARCOM, 2000) and the 2005 European Commission. Currently several studies have shown NP accumulation within sewage sludge, with the possibility that high concentrations can enter ⇑ Corresponding author. Tel.: +86 010 82106567; fax: +86 010 82106568. E-mail address: [email protected] (J. Wang). 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.09.131

the environment following application of sludge to agricultural land (Ahmed, Javed, Tanvir, & Hameed, 2001; Staples, Mihaich, Carbone, Woodburn, & Klecka, 2004). Food products were analysed also as a potential source of NP and NPEO in the human nutrition. According to a German study (Guenther et al., 2002) analysing 60 different foodstuffs purchased from supermarkets in Germany, NP is ubiquitous in foods; the levels found ranging from 0.1 to 19.4 lg kg 1 on the fresh weight basis. Therefore, determination of the concentration of NPEO and their metabolites in vegetables and crops is of primary importance because of the threat of these compounds to human health. Numerous methods for the determination of NPEO and their metabolites in various matrices such as soil, drinking water, atmosphere and foodstuffs have been reported. Most of the analytical techniques available have been based on HPLC (Gadzala-Kopciuch, Filipiak, & Buszewski, 2008; Langford et al., 2007), GC/MS (Li, Cheng, & Ding, 2008; Yang & Ding, 2005), LC–MS and LC–MS/MS (Schröder, 2001; Trenholm, Vanderford, Holady, Rexing, & Snyder, 2006). Currently, few methods using LC–MS/MS are knowed simultaneously analysis of the residues of 4-NP, NP1EO and NP2EO in complex foodstuffs matrices. Therefore, it is necessary to develop a novel analytical method for the isolation, identification and quantification of NPEO and their metabolites in foodstuffs matrices.

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Sample pretreatment plays an important role in the analysis of multi-component NPEO residues. Current pretreatment approaches mainly utilises solid-phase extraction (SPE), pressurised liquid extraction (PLE), accelerated solvent extraction (ASE) and microwave-assisted extraction (MAE) to achieve the enrichment of these residues (Fountoulakis et al., 2005; Takino, Daishima, & Yamaguchi, 2000; Zhang, Hibberd, & Zhou, 2006). Due to lack of advanced pretreatment instrument in some developing countries, these methods are less widely used. The main goal of this work was to develop and optimise liquid–liquid extraction (LLE) combined with SPE for the extraction of 4-NP, NP1EO and NP2EO from vegetable and crop samples. In this study, LC–MS/MS was used as the analytical technique. Parameters of the extraction, cleanup and mass detection were optimised. Using the optimised conditions enabled the successful determination of all three compounds (4NP, NP1EO and NP2EO) in vegetables and crops.

2. Experimental 2.1. Reagents and materials 4-NP, 4-n-NP (it is used as internal standard), NP1EO and NP2EO standards were purchased from Tokyo Chemical Industry (Tokyo, Japan). Methanol, formic acid, toluene and ammonium acetate were of HPLC grade from Fisher Scientific (Nepean, Ontario, Canada). Water was prepared on a Milli-Q purification system from Millipore (Bedford, MA, USA). All glassware was solvent rinsed before use. SPE was performed using a Supelco Dual Layer ENVI™ equipped with a carbon black amino-group SPE column and ENVI™ Carb II/ PSA column (Bellefonte, PA, USA). 2.2. Preparation of standard solutions Stock solutions of each analyte (1000 lg mL 1) were prepared in methanol. Then, each stock solution was diluted to 5 lg mL 1 with methanol, and stored at 20 °C. Mixtures of the analytes for the preparation of working standards and for sample fortification were prepared in a mobile phase of methanol/10 mM ammonium acetate (90:10, v/v). All stock solutions and mixtures were stored in the dark at 4 °C and analysed as soon as possible. 2.3. Sample preparation For this study, four types of fresh vegetables (cabbage, pakchoi cabbage, leek and cucumber) and two types of crops (maize and soy bean) typically consumed by Chinese were tested. All samples were purchased at the same time from local supermarkets. Samples were refrigerated in their original packaging at 20 °C and analysed within a week after purchase. The samples were chopped into small pieces, and then were ground using a food blender (Jiuyang Corporation, china) just before the extraction. Accurately weighed a 25 g fresh ground sample from above six types of samples was placed into a 150 mL beaker. A volume of 50 mL of acetonitrile was added to the sample, and then homogenised using a tissue homogenizer for 2 min. The mixture was filtered through a glass funnel. The filtrate was collected into an extraction vessel with a screw cap.5–7 g of NaCl was added and then was shaken intensely. After standing 10 min at room temperature (20 °C), 25 mL of the acetonitrile upper layer liquid in the phase separation solution was taken to dryness in a rotary evaporator under a nitrogen stream at 40 °C. Then, the residues were dissolved with 3 mL of acetonitrile/toluene (3:1), and sonicated for 5 min for further purification.

The ENVI™ Carb II/PSA SPE cartridge was first conditioned with 15 mL acetonitrile/toluene (3:1). Then 3 mL of the pretreated sample was loaded onto the SPE cartridge. The column was airdried and eluted with 25 mL acetonitrile/toluene (3:1). The eluate was evaporated to dryness, and then re-dissolved with 5 mL of methanol containing the internal standard (4-n-NP). Finally, the re-dissolved solution was directly injected into the HPLC–MS/ MS system. 2.4. HPLC conditions HPLC analysis was performed on a liquid chromatograph Surveyor, TSQ quantum mass spectrometer system (Finnigan, USA). A Phenomenex Luna C18, 3 lm, 2.0 mm i.d  150 mm column (Phenomenex USA) was utilised as the analytical LC column. The chromatographic separation was performed using isocratic conditions with a mobile phase of methanol/10 mM ammonium acetate (90:10, v/v). The flow rate was 0.2 mL min 1 while the injection volume was 10 lL. Moreover, to obtain good precision, the column and sample temperature were maintained with the LC auxiliary devices at 30 °C and 20 °C, respectively. 2.5. MS/MS conditions MS/MS was performed on the Finnigan TSQ quantum discovery triple quadrupole mass spectrometer equipped with an ESI source (Micromass, UK). The parameters used for the mass spectrometer were as follows: (1) Mass spectrometry parameters for NP1EO and NP2EO (ESI+): source voltage 4500 V, spray gas 0.60 L/h, auxiliary gas 0.56 L min 1, capillary compensation voltage 35 V, collision argon 0.01 L/h, and the capillary temperature 250 °C, (2) Mass spectrometry parameters for 4-NP and 4-n-NP (ESI ): source voltage 3000 V, spray gas 0.70 L/h, auxiliary gas 0.6 L min 1, capillary compensation voltage 35 V, collision argon 0.03 L/h, and capillary temperature 350 °C. The parameters for the m/z and collision energy of parent ions and quantitative product ions from NP, NP1EO and NP2EO are shown in Table 1. 3. Results and discussion 3.1. Optimisation of LC conditions The optimal ionisation largely depends on the LC mobile phase composition. Considering the characteristics of electrospray ionisation and the chemical properties of 4-NP, 4-n-NP, NP1EO and NP2EO, a series of preliminary experiments were performed to test which additives (acetic acid, ammonium acetate) provided the optimum response for the mobile phases containing acetonitrile and methanol. The result showed that signals for 4-NP and 4-n-

Table 1 MS/MS Parameters for the determination of 4-NP, 4-n-NP, NP1EO and NP2EO.

*

Compound

Parent mass (m/z)

Daughter mass (m/z)

Collision energy (eV)

4-NP

219

4-n-NP NP1EO

227 282

NP2EO

326

133* 147 173 112 127* 139 71 183* 121

27 21 10 18 9 13 9 7 20

For quantitative ions.

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NP were better resolved when analysed in a mobile phase consisting of methanol and 0.1–0.2% acetic acid in the water, while NP1EO and NP2EO produced signals of low intensity under these conditions. More than once experiments, the results finally showed the addition of ammonium acetate to the mobile phase not only improved the ionisation efficiency of 4-NP and 4-n-NP but also improved that of NP1EO and NP2EO. However, higher ammonium adduct concentrations resulted in low detection sensitivity of 4-NP, 4-n-NP, NP1EO and NP2EO. Considering the combined factors between separation and ionisation efficiency, we finally chose methanol/10 mM ammonium acetate (9:1) as the mobile phase.

analytes produced one or two fragment ions, and the second mass spectrum was very simple; thus, only one transition was selected for multiple reaction monitoring (MRM). The total ion chromatograms and the quantitative ion chromatograms for these compounds are shown in Figs. 1 and 2, respectively. When the parent ion and product ion of the target compounds were obtained, parameters such as capillary voltage, temperature, skimmer offset and dwell time were optimised. The result showed that, as for NP2EO, the sensitivity was improved when the dwell time was set at 0.1 s, while a dwell time of 0.3 s was optimal for 4-NP, 4-n-NP and NP1EO.

3.2. Optimisation of mass spectrometer conditions

3.3. Sample pretreatment optimisation

For the selection of the parent ions of the compounds of interest, different ionisation modes were used according to the chemical ionisation characteristics of each compound. Transitions from the [M+NH4]+ ions for NP1EO and NP2EO, and transitions from the [M H] ion for NP and 4-n-NP were monitored. For this study, the [M+NH4]+ ion for NP1EO and NP2EO were chosen because the molecular ion was of low intensity when no modifier was added to the mobile phase. The most intense ion was the sodium adduct, but it could not be used with great efficiency in the MS/MS mode, as it was too stable to give relevant fragmentation. This was why we forced the formation of the ammonium adduct by incorporating ammonium acetate in the mobile phase. The [M H] of NP and 4-n-NP (internal standard) were stronger in intensity in the optimised mobile phase (methanol/10 mM ammonium acetate) in the negative ion mode. As a result, NP, 4-n-NP, NP1EO and NP2EO were identified as m/z 219 [M H] , m/z 227 [M H] , m/z 282 [M+NH4]+, m/z 326 [M+NH4]+ as the parent ions of collision induced dissociation, respectively. For the optimum conditions, the precursor ions and fragment ions are shown in Table 1. All target

Extraction and sample clean-up preparation strategies for 4-NP, NP1EO and NP2EO in food samples mainly involve an extraction with organic solvent following SPE. In this work, preliminary studies were conducted to optimise the solvents and extraction conditions. The extraction efficiency of the compound residues from vegetable and crop samples was studied using two solvents: acetonitrile and dichloromethane. The results obtained showed that acetonitrile provided slightly higher recoveries (88–90%) of the compounds than that (68–70%) of dichloromethane. Based on these results, acetonitrile was selected as the extractant for pretreatment. Due to the fact that the compounds exist in the matrix at trace quantities in the presence of various other compounds, further cleanup and enrichment procedures using solid phase extraction (SPE) have been developed in this study. Based on several other well-developed methods for the determination of the degradation products of NPEOs (Lu, Chen, Sung, Wang, & Mao, 2007; Schröder, 2001; Wang, Pan, Liu, & Fei, 2009), two types of SPE cartridges (an Oasis HLB cartridge and ENVI™ Carb II/PSA SPE cartridge) were tested. The results showed that higher recoveries

RT: 4.04 AA: 173791 SN: 921

RT: 0.00 - 6.00SM:15G 100

NL: 1.51E4 TIC MS Genesis 14-std-509

95 90 85 80 75

Relative Abundance

70 65 60 55 50 45 40 35 30 25

RT: 4.77 AA: 19040 SN: 158

20 15 10 5 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Time (min) Fig. 1. The total ions chromatograms of 4-NP, NP1EO, NP2EO and 4-n-NP in spiked sample.

5.5

6.0

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RT: 4.03 AA: 347561 SN: 931

100 80 60 40 20 0

RT: 4.78 AA: 37501 SN: 20380

100 80 60 40 20 0

RT: 3.86 AA: 4338744 SN: 728

100 80 60 40 20 0

RT: 3.94 AA: 121787027 SN: 16111

100 80 60 40 20 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Time (min) Fig. 2. The extracted ion chromatogram of four compounds in sample spiked with a standard mixture of 5 lg L NP2EO).

(>80%) of 4-NP, NP1EO and NP2EO were obtained by using the ENVI™ Carb II/PSA SPE cartridge. However, when a similar range of sample volumes was used for evaluating the performance of the HLB cartridge with the best extraction conditions, a low recovery (50–60.8%) of NP2EO from vegetable and crop samples was obtained. Therefore, a Carb II/PSA SPE cartridge was selected as the SPE column for this study. In addition, different elution solvents and their volumes for this SPE were further optimised. The optimisation results indicated that a volume of 25 mL with a solution of acetonitrile/toluene (3:1) was sufficient to elute most of the target compounds from the cartridge (65–118% recoveries of six types of samples). In this study, residues of 4-NP in the blank samples were observed. Several procedures were used to eliminate this contamination in the present work. We finally found that the source of this constant contamination was from microporous filtration. Therefore, we did not use microporous filtration for the detection of 4NP in foodstuff samples. 3.4. Method validation Usually the quantification of drug residues is performed using a matrix-matched calibration curve made from fortified blank samples prepared in the same matrix as the real samples. To test the linearity of the calibration curve, the mixed NPEOs standard solution, with the concentration sequence of 5, 10, 15, 20, 50, 100 and 200 ng mL 1 each for 4-NP and NP1EO and 0.5, 1, 2, 5, 10, 50 and 100 ng mL 1 for NP2EO, was added to the blank organic vegetable and crop samples (from Tibetan organic crops farm), and 2 ng mL 1

1

for 4-NP (4-n-NP) and 5 lg L

1

for NP1EO (0.5 lg L

1

for

of 4-n-NP (internal standard solution) was also included. Good linear response and good coefficients of determination (r2 = 0.99) were achieved over the concentration range. The precision of the method was determined by spiking blank organic vegetable and crop samples with the target compounds at 10, 15 and 20 lg kg 1 for 4-NP and NP1EO (2, 5 and 10 lg kg 1 for NP2EO). Six replicate analyses were performed at each concentration. The recovery of these compounds spiked at different concentrations in various matrices is summarised in Table 2. All of target compounds were recovered (65–118%) in the different spiked samples and the RSD values at each validation level ranged from 0.7% to 6.8%. Furthermore, for each analyte, in the intra-day and inter-day reproducibilities were determined by testing five replicates independently, with samples extracted at 5 and 10 lg kg 1 for 4-NP and NP1EO (0.5 lg kg 1 for NP2EO) respectively. The repeatability was good both on an inter-day and intra-day cycle. The repeatability values, given as RSD, were in the range of 1.1–6.2% for the inter-day evaluation and of from 2.5– 8.0% for the intra-day test. The limits of detection (LOD) and quantification (LOQ) of the method were experimentally estimated from the analysis of vegetables and crops at the minimum concentration of each analyte, giving a signal to noise ratio of 3 and 10, respectively. The average LODs obtained in various matrices was 3 and 5 lg kg 1 for 4-NP and NP1EO respectively, and 0.1 lg kg 1 for NP2EO. The average LOQs, which were approximately three times higher than the corresponding LODs, were determined to be 10, 15 and 0.5 lg kg 1 for 4-NP, NP1EO and NP2EO, respectively. The effect of different matrices on the LODs and LOQs of the target compounds was negligible.

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Table 2 Recovery of three compounds in spiked vegetables and crops (n = 6). Samples

4-NP

NP1EO 1

Added (lg kg Vegetables

Pakchoi

Cabbage

Leek

Cucumber

Crops

Soy bean

Maize

Recovery (%)

Added (lg kg

10 15 20 10 15 20 10 15 20 10 15 20

99.11 ± 1.2 85.10 ± 1.8 97.00 ± 2.4 65.88 ± 5.0 75.25 ± 4.4 111.21 ± 6.8 79.88 ± 1.4 85.14 ± 0.8 76.45 ± 2.8 109.45 ± 2.1 100.3 ± 1.4 96.8 ± 1.5

10 15 20 10 15 20

84.76 ± 3.8 75.99 ± 3.4 88.97 ± 2.1 87.65 ± 1.8 84.10 ± 0.9 94.07 ± 1.5

)

Table 3 Concentration of three target compounds detected in vegetables and crops (fresh weight). Average concentration (lg kg

Samples

4-NP

NP1EO

Vegetables

Pakchoi Cabbage Leek Cucumber

21.46 ± 3.5 21.59 ± 2.3 9.70 ± 2.8 33.39 ± 1.4

n.d n.d n.d n.d

Crops

Soy bean Maize

37.61 ± 2.5 72.5 ± 1.9

n.d n.d

*

1

NP2EO 1

1

Recovery (%)

Added (lg kg

10 15 20 10 15 20 10 15 20 10 15 20

118.1 ± 1.2 98.50 ± 3.0 87.88 ± 1.4 101.2 ± 3.4 85.61 ± 3.5 79.59 ± 3.3 91.45 ± 1.8 89.78 ± 2.8 98.71 ± 2.5 97.85 ± 0.7 100.2 ± 1.8 87.97 ± 2.7

2 5 10 2 5 10 2 5 10 2 5 10

65.24 ± 5.8 67.58 ± 2.9 84.42 ± 1.4 67.24 ± 6.0 77.56 ± 4.8 78.24 ± 3.8 113.45 ± 2.9 102.4 ± 1.0 75.8 ± 3.0 101.45 ± 2.8 93.54 ± 1.3 89.99 ± 1.4

10 15 20 10 15 20

65.34 ± 3.8 68.42 ± 2.8 78.01 ± 2.9 81.44 ± 0.8 98.10 ± 1.9 87.65 ± 1.8

2 5 10 2 5 10

65.86 ± 1.8 78.01 ± 2.4 68.99 ± 3.0 101.07 ± 1.2 98.97 ± 1.9 100.41 ± 1.0

)

)

Recovery (%)

onto the water samples. Taking above the results into account, the presence and levels of the NPEOs metabolites in agricultural products should be a matter of concern.

,fresh weight)* NP2EO 0.83 ± 1.9 0.65 ± 2.5 0.70 ± 3.3 0.86 ± 2.1 0.11 ± 1.0 15 ± 2.5

n.d: Lower than the detection limit (LOD). Average from n = 3 separate experiments.

*

The analytical method developed was used to monitor the presence of 4-NP, NP1EO and NP2EO in vegetables and crops collected from different supermarkets. Positively identified target compounds in fruit and vegetable samples were analysed three times. Table 3 lists the concentrations of 4-NP and NP2EO that were detected in all the selected vegetable and crop samples. The results indicate that 4-NP and NP2EO were detected in all the samples but NP1EO residue was not detected in all selected vegetables and crops. The varying concentrations of these compounds in different vegetables and crops revealed that most of the samples may be contaminated by NPEOs from miscellaneous pathways and at different stages of the food production process. Some may have originated from alkylphenol ethoxyl (APEOs), which were used as nonionic surfactants in disinfectants or as emulsifiers in pesticide formulations. Following their use in agriculture, the degradation products of APEOs could promote the accumulation of 4NP, NP1EO and NP2EO on the root and other parts of the vegetables and crops. Another possible source might be from the plastic foodcontact materials where degradation products such as NP residues from tris(nonylphenol) phosphate, which is a component of plastic, may migrate into vegetables when used in food-contact applications (Inoue et al., 2001; Lim, Kwack, Kim, Kim, & Lee, 2009; Loyo-Rosales, Rosales-Rivera, Lynch, Rice, & Torrents, 2004; Votaˇ ízˇková, 2009). In this work, we had also vová, Dobiáš, Voldrˇich, & C detected various concentrations of NP in drinking water samples if the water passed through filter films before injection into the MS/ MS. It is likely that the filter films contained these degradation products or impurities of 4-NP residues which were then passed

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